The invention relates to a domestic appliance, in particular a water-carrying domestic appliance, e.g. a dishwasher, containing a moving object to be carried by a plain bearing, with a coating being disposed on the moving object.
In water-carrying domestic appliances, e.g. in dishwashers or washing machines, “wet-running pumps” are used. The shaft bearing is designed as a radial plain bearing, which the relevant washing liquid, essentially water, flows over in wet-running pumps. The washing liquid, as the medium to be pumped by the pump, often contains corrosive substances, e.g. chloride from domestic or drinking water or produced by the water softener of the dishwasher. Hence stainless steel shafts are often used to avoid corrosion on the shaft. The washing liquid frequently contains particulates such as sand, silicates or metals, some of which are abrasive components originating from particulates removed e.g. from the bearing or the shaft. Stainless steel shafts, however, have a low surface hardness, so that when corresponding particulates exist in suspension in the liquid this results in roughening of the surface of the stainless steel shaft and hence to premature failure of the radial bearing owing to increased friction.
When hardened steels are used, failure generally occurs from corrosion because hardened steel is a carbon alloy. Even applying a hard-metal coating such as chrome by electroplating, for example, results in failure of the radial bearing because of flaking. Shafts made of oxide-ceramic materials are more expensive to manufacture and are only of limited use because the properties of oxide-ceramic materials make them difficult to shape and machine. In addition, shafts made of oxide-ceramic materials cannot withstand dry running in radial bearings.
Hence the object of the present invention is to provide a domestic appliance having a low-cost drive shaft that is also capable of working with liquids containing corrosive substances and particulates in a wet bearing and has good dry-running properties.
It is also the object of the invention to provide a suitable coating method.
This object is achieved by a domestic appliance according to the invention as claimed in claim 1 and by a coating method according to the invention as claimed in claim 20. Advantageous embodiments are the subject of the dependent claims.
In a domestic appliance according to the invention, e.g. hairdryer, mixer or vacuum cleaner, in particular a water-carrying domestic appliance, e.g. dishwasher or washing machine, containing a moving object to be carried by a plain bearing, with a coating, in particular containing a metallic element, being disposed on the moving object, the coating contains at least one non-metallic element having a mole fraction of at least 1%, where the hardness of the coating equals at least 800 HV, in particular at least 2000 HV.
In particular, the coating contains at least one metallic element. The mole fraction of the non-metallic element or of the non-metallic elements, e.g. nitrogen, is greater than 3%, for example, in particular greater than 25%. Materials used for coatings often contain traces, i.e. very small proportions, e.g. in the region of less than 0.5% mole fraction, of elements, e.g. oxygen or carbon. In particular the coating is made of ceramic or ceramic-like materials. The coating is preferably made of a chemical compound containing at least one non-metallic element and at least one metallic element.
The at least one non-metallic element is preferably, for example, an element from the halogen group, from the oxygen group or from the nitrogen group or carbon. Non-metallic elements are taken also to include noble gases.
In particular, the coating contains at least one metallic element having a mole fraction of at least 1%.
The metallic elements are preferably, for example, elements from the group of alkali metals, alkaline earth metals or earth metals or, for example, lead or tin.
The coating material is preferably a chemical compound made of at least one metallic element and at least one non-metallic element.
The coating is preferably applied by physical deposition from the gas phase (PVD technique) e.g. by vapor deposition, sputtering, the plasma vacuum technique or ion implantation.
In a further embodiment, the coating is applied by deposition from the vapor phase, e.g. thermal CVD, plasma CVD, photon CVD or laser-induced CVD.
In an additional embodiment, the coating is applied by chemical coating techniques e.g. electroplating.
The coating preferably contains chrome and/or nitrogen and, in particular, is made of a chrome-nitrogen compound. The metallic element is hence chrome and the non-metallic element nitrogen. Other chemical elements can also be used apart from these elements. For instance, the coating may be made of titanium nitride, although owing to the higher degree of hardness compared with a chrome-nitrogen compound this tends to flake off and is hence less suitable.
The thickness of the coating preferably lies between 0.1 μm and 20 μm, in particular in the region of 5 μm.
In a preferred embodiment, the moving object is a shaft that is carried by a radial bearing.
In a preferred embodiment, the shaft can be driven by an electric motor.
In a further embodiment, an impeller or a propeller of a pump can be driven by the shaft.